Irradiation method of particle beam therapy system is divided broadly into two methods. That is, a broad irradiation method in which a beam is irradiated into whole of patient's affected area simultaneously and a scanning irradiation method in which a beam is scanned and irradiated. A scanning irradiation method includes a spot scanning irradiation method and a raster scanning irradiation method, however, in this specification, these methods will be referred to collectively as scanning irradiation methods. In order to realize a scanning irradiation method, equipment and controlling methods which are suited for the irradiation methods are required. It is necessary to devise an end from which a particle beam is actually irradiated so as to realize a scanning irradiation method. An end for irradiating a particle beam is referred to as irradiation system, irradiation field forming apparatus, irradiation head, irradiation nozzle, etc.
Regarding irradiation systems for realizing scanning irradiation methods, in order to increase irradiation position accuracy of an affected area of a patient, it is necessary to irradiate a beam having a small beam size. On the other hand, when a beam travels in the atmosphere a size of beam is increased by scattering. Therefore, a configuration of system in which scattering of a beam is suppressed, a part in which a vacuum region or a region of gas such as helium which is lighter than air is secured so as to decrease a beam size is proposed (for example, Patent Document 1). In this specification, a part, in which a vacuum region or a gas region is secured, will be referred to as a duct. Further, in a duct, a part where a particle beam passes will be referred to as a window, and in this specification, a window which is provided in the most downstream of a particle beam track will be referred to as a beam outlet window.
An irradiation nozzle of a particle beam therapy system for realizing a scanning irradiation method includes a vacuum duct for securing a vacuum region, a window where a particle beam passes in a vacuum duct (beam outlet window), a beam scanning apparatus for scanning a particle beam, a beam position monitor for monitoring abeam dose, and a dose monitor for monitoring abeam dose, etc.
When a beam which travels in a straight line hits an obstacle, scattering is generated, and the beam propagates with a certain spread. The spread refers to as a scattering angle, and is indicated as θ (radian). A diameter of a beam spot which is away from a distance of r from an obstacle is approximately rθ. In an irradiation nozzle of a scanning irradiation method, a window which is provided more downstream than a scanning electromagnet or a beam position monitor corresponds to an obstacle. That is, a particle beam is scattered at a part of the window, and then the particle beam propagates with a certain spread.
According to conventional technology, an obstacle which is a factor of beam scattering is positioned away from an isocenter which is an irradiation point, that is, a distance of r is large, therefore, a beam spot diameter is large. Consequently, a beam size which is not small enough for performing practical scanning irradiation method could not be obtained.
In order to solve the above-mentioned problem, Patent Document 2 discloses a configuration, that is, by making a vacuum duct to be expandable, a member, which is a factor of scattering including a beam outlet window which is provided at an end of a vacuum duct, a beam position monitor, a dose monitor, etc., is approached to a patient and a particle beam is irradiated onto the patient.